CHICAGO — Cook County’s sheriff filed a lawsuit Thursday against Craigslist, saying the popular online classifieds site not only allows the solicitation of prostitution but has actively created “the largest source of prostitution in America.”

“They’ve actually catered their site so it facilitates (prostitution), where you can actually and more specifically and quickly get to what you want,” said Sheriff Tom Dart at a news conference announcing the federal lawsuit.

“How is that different than somebody who’s aggressively and actively working with a pimp to try to get the word out about the women working for him?” he said.

Craigslist spokeswoman Susan Best said that although the company had not seen the lawsuit, it vehemently disputes the sheriff’s contentions. The company cooperates with law enforcement, has taken several steps to prevent illegal use of the site and pulls inappropriate ads, she added.

“Misuse of Craigslist to facilitate criminal activity is unacceptable, and we continue to work diligently to prevent it,” Best said in an e-mail to The Associated Press.

But Dart said Craigslist has refused over the last two years to prevent people from posting the ads that pop up by the thousands in its “Erotic Services” section.

He acknowledged the company does warn that solicitation of prostitution is prohibited, but said it ignores obviously illegal ads.

“None of these ads require any imagination, there’s no mystery,” Dart said, pointing to ads with phrases such as “Teens for cash … $100 quickie,” and “Ask me about my 2 girl specials.”

Dart brought to the press conference a 19-year-old woman who he said turned to prostitution — and was recently arrested for allegedly soliciting an undercover officer — after originally going to the site in search of modeling jobs.

Craigslist has created a public nuisance and is in violation of city, county, state and federal prohibitions, Dart says in the lawsuit. He asks that a federal judge shut the “Erotic Services” section down.

Like other sites, Craigslist, which allows users to post classified ads and other items, generally doesn’t check the postings or remove them unless it receives complaints. Federal law offers broad immunity to service providers for content posted by users, as long as they respond to specific complaints.

The previous lawsuits failed because the defendants’ only role was allowing ads, Dart said. In contrast, he argued, the purpose of the “Erotic Services” is to facilitate prostitution.

“They are not passive,” Dart said. “They are actively involved with this.”

Further, he said his own officers’ experience — in which they posed as minors seeking sex — demonstrates that Craigslist does not look very hard for illegal activity.

“We put up … one saying 14-year-old looking for sex,” he said. “That ad wasn’t taken down. It sat out there until we took it down.”

There have been several arrests around the nation that stemmed from “Erotic Services” ads posted on Craigslist.

In New York, federal prosecutors charged a New York man in November with being a violent pimp and alleged he advertised the services of girls and women between the ages of 15 and 20. And in January, two Wisconsin women were charged with misdemeanor prostitution after allegedly offering sex for money on Craigslist.

Craigslist reached an agreement in November with attorneys general in Connecticut, Illinois and several other states that called for the company to crack down on prostitution ads. The Connecticut attorney general’s office had contacted the site after receiving several complaints about photographs depicting nudity.

Jim Buckmaster, Craigslist’s CEO, said at the time that the agreement would allow legitimate escort services to continue advertising while discouraging illegal activity by requiring anyone posting “Erotic Services” ads to provide a working phone number and pay a fee with a valid credit card.

Craigslist also agreed to provide that information to law enforcement if subpoenaed.

At the time, Connecticut Attorney General Richard Blumenthal, who brokered the agreement, said Craigslist cooperated fully.

Also in November, Craigslist filed lawsuits against more than a dozen software and Internet companies that help people who post erotic service ads to circumvent the Web site’s defenses against inappropriate content and illegal activity.

On Thursday, Best pointed to that agreement as evidence of Craigslist’s diligence.

But the lawsuit has prompted Blumenthal to evaluate the agreement.

“If my office determines these measures have failed to stop prostitution and human trafficking, I will seek much stronger safeguards and consider other steps,” he said in a statement, adding that Craigslist is cooperating.